Spherically aggregated boron nitride (BN) is advantageous for improving thermal conductivity. However, it is difficult to process due to its fragility and poor wettability. Herein we report coating boron nitride aggregates with polysilazane (PSZ) and three different silanes, glycidoxypropyltrimethoxysilane (GPTMS), aminopropyltriethoxysilane (APTES), and mercaptopropyltrimethoxysilane (MPTMS), via hydrolysis and condensation. The PSZ coating protected BN from external force, and the silane coatings enhanced interfacial adhesion between the filler and the matrix. A composite prepared with APTES had the highest thermal conductivity of 11.8 W m^-1 K^-1 at a filler fraction of 75 wt%. Its thermal conductivity was 62 times higher than that of the neat epoxy.